Gangway : Validating incoming messages.

Implementing plumbing to validate incoming Gangway messages.
Validation occurs in the processing of Liberator's malformed? decision
point.

worker.clj
-- Added valid-msg? function. Initial implementation simply confirms
that message can be parsed into JSON string. Data "shape" validation
to be implemented next.

web.clj
-- Added :malformed? and :handle-malformed keys to the incoming!
resource. The function defined for :malformed? calls in to
gangway.worker/valid-msg? function.

--Tom